1. Field of the Invention
The present invention concerns the technical field of imaging methods for generation of two-dimensional (2D) slice images. In particular it concerns a method for determination of an acquisition sequence for acquisition of slice plane data of slice planes of an examination subject for an imaging method for generation of slice images belonging to the slice planes.
2. Description of the Prior Art
Data acquisitions from an examination subject in the form of two-dimensional slice images are generated in the examination of patients with a magnetic resonance tomography apparatus. Typically, but not necessarily, the slices images are parallel to one another in order to acquire three-dimensional views of the examination subject in this manner. A number of individual measurements are implemented for each slice image, with the resonance data of the slice planes being acquired line-by-line. In the implementation of an examination, a single measurement completely occupies the magnetic resonance tomography apparatus so that only one measurement can ever be implemented in each open time interval.
Before beginning an examination (i.e. the generation of slice images of an examination subject) the examination subject and special, examination-specific parameters are established by an operating personnel, for example a radiologist or a medical-technical assistant. The operator establishes the slice planes (“spatial slices” or “slices”) to be measured, the time duration of the individual measurements and the time interval between the individual measurements. The acquisition then ensues in a computer-controlled manner, with individual measurement sequences being automatically arranged (according to acquisition scheme) into an acquisition sequence for acquisition of the resonance data of all slice planes. The slice images belonging to the slice planes are generated from the resonance data by Fourier transformation.
As is generally known, magnetic resonance tomography is based on the fact that protons and some atomic nuclei exhibit a magnetization that can be aligned by the application of an external magnetic field. If a radio-frequency pulse of suitable frequency is applied perpendicular to the magnetic field, the directed magnetization can be deflected, and a change of a magnetization component parallel to the magnetic field direction (“longitudinal magnetization”) and perpendicular to the magnetic field direction (“transverse magnetization”) can be caused. The longitudinal magnetization subsequently returns again to the equilibrium state due to spin-grid interactions, which can be described with a time constant T1. The transverse magnetization decays with a characteristic decay time (described by the time constant T2) due to spin-spin interactions. Tissue properties can be characterized by the two time constants T1, T2 since the resonance signal that is emitted from the body and acquired by measurement apparatuses is dependent on the structure of the tissue, in particular the proton density.
Today two different measurement methods are commonly used for examinations in magnetic resonance tomograph, namely the TSE (Turbo Spin Echo) measurement method and the IRTSE (Inversion Recovery Turbo Spin Echo) measurement method, in which different pulse sequences are used.
In a spin echo pulse sequence a spin system is initially excited via a 90° radio-frequency pulse so that the excited protons precess in phase (phase synchronization) and a transverse magnetization occurs that rotates around the applied external magnetic field with the Larmor frequency. Spin-spin interactions lead to dephasing of the precession movements and thus to T2 relaxation. Before equilibrium is reached, the magnetization is tilted again by a 180° radio-frequency pulse, causing the spins to be phase-synchronized again. The resonance signals are subsequently acquired.
In an inversion recovery pulse sequence, a 180° radio-frequency pulse (“inversion pulse”) is initially radiated in order to invert the longitudinal magnetization (anti-parallel position to the external magnetic field). The spins thus are not phase-synchronized, so no transverse magnetization thus arises. Due to the spin-grid relaxation the longitudinal magnetization returns again to equilibrium with the time constant T1. Before the longitudinal magnetization reaches the thermal equilibrium, a 90° radio-frequency signal (“measurement pulse”) is radiated by which the momentary magnetization is tilted by 90° (thus perpendicular to the external magnetic field) and thus is converted into a transverse magnetization. The spins now precess in phase and relax with the time constant T2. The resonance signals are subsequently acquired.
It is significant in the IRTSE pulse sequence that a time span that is generally constant for an examination exists between the two radiated radio-frequency pulses. This procedure can be repeated multiple times, with the time between two inversion pulses of the same slice plane being designated as a repetition time. The repetition time should be large enough so that the magnetization is completely repeated at the beginning of a new measurement for the same slice plane.
The IRTSE measurement sequence is now explained in detail with regard to FIG. 1, wherein a single IRTSE measurement sequence of a nuclear magnetic resonance tomography apparatus is exemplarily presented in a schematic manner. The IRTSE measurement sequence shown in FIG. 1 has inversion phase I1 for emission of the inversion pulse with a time duration TA=1 time unit and a measurement phase M1 (composed of two measurement sub-phases M11, M12) with a total time duration TE+TM=3 time units. The first measurement sub-phase M11 which lasts TE=1 time unit serves for the emission of the measurement pulse. The second measurement sub-phase M12 which lasts TM=2 time units serves for the actual measurement for acquisition of the resonance signals. In this example the measurement phase M1 encompasses only a single measurement, but further measurements that each occupy two time units could ensue in further measurement sub-phases. A time span R1 with a time duration TGAP of 5 time units (recovery phase) lies between the inversion phase I1 and the associated measurement phase M1.
Various acquisition schemes, which can be classified as “sequential”, “overlapping” and “distributed” acquisition schemes, are known for the acquisition of resonance data of 2D slice planes for generation of 2D slice images in an IRTSE imaging method by nuclear magnetic resonance tomographs (see, for example, Matt A. et al., “Handbook of MRI Pulse Sequences”, Elsevier, Boston 2004, section 14.2.2, pages 611 and the following). The acquisition schemes known in the prior art are now explained in detail with reference to FIG. 2 through 4.
FIG. 2 is considered first, wherein a sequential acquisition scheme is presented. The sequence acquisition of resonance data is based on the principle that measurement sequences are implemented without an inversion phase or measurement phase of another measurement sequence being implemented in the recovery phase of a considered measurement sequence (which recovery phase lies between the inversion phase and the measurement phase). As is illustrated in FIG. 2, a first measurement sequence is first implemented which comprises inversion phase I1 and measurement phase M1 with a recovery phase R1 situated in-between. The measurement phase M1 itself is composed of a number of individual measurements, as is illustrated by the vertical lines. A further measurement sequence subsequently follows comprising inversion phase I2 and measurement phase M2 with an intervening recovery phase R2 and so forth. A number of measurement sequences are implemented in this manner until all resonance data from all slice planes of the examination subject are acquired. The resonance data of various slice planes are acquired in adjacent measurement sequences in order to adhere to the required repetition times between two measurements of the same slice plane. A disadvantage in the sequential acquisition scheme is the large time requirement for the complete acquisition of the resonance data since the recovery phase between inversion phase and measurement phase elapses as “unused” time.
An acquisition scheme superior to the sequence acquisition scheme in this regard is the overlapping acquisition scheme which is illustrated in FIG. 3. The overlapping acquisition of image data is based on the principle that a further inversion phase of the subsequent measurement sequence is implemented in the intervening recovery phase in each measurement sequence. This is exemplarily illustrated for two measurement sequences in FIG. 3. According to FIG. 3, an inversion phase I1 of a first measurement sequence is implemented first, followed by an inversion phase I2 of a following second measurement sequence within the recovery phase R1 of the first measurement sequence. After expiration of the recovery phase R1 of the first measurement sequence, a measurement phase M1 of the first measurement sequence follows within the recovery phase R2 of the second measurement sequence. The measurement phase M1 of the first measurement sequence has a first measurement sub-phase M11 for emission of the measurement pulse (measurement preparation) and two further measurement sub-phases M12, M13, namely two individual measurements for acquisition of the resonance signal. A measurement phase M2 (comprising three measurement sub-phases M21, M22, M23) of the second measurement sequence follows this. The first measurement sub-phase M21 serves in a corresponding manner for the emission of the measurement pulse while the second and third measurement sub-phases are two individual measurements for acquisition of the resonance signal.
Two further inversion phases subsequently follow, followed by two further measurement phases and so forth until the resonance data of all slice planes are acquired. In the number example shown in FIG. 3 16 time units are necessary for implementation of two measurement sequences with the overlapping acquisition scheme when the inversion phase takes one time unit, the recovery phase takes five time units and the measurement phase takes five time units in total in every measurement sequence. In order to execute these four individual measurements with the sequential acquisition scheme, this would have to be executed twice with two individual measurements each, with a total time requirement of 22 time units.
The distributed acquisition scheme which is illustrated in FIG. 4 offers a further improvement in this regard. The distributed acquisition of resonance data is based on the principle that a measurement phase of the preceding measurement sequence and an inversion phase of the subsequent measurement sequence are implemented in the intervening recovery phase in each measurement sequence. This is exemplarily illustrated for a number of measurement sequences in FIG. 4. According to FIG. 4, at least one inversion phase I1 of a first measurement sequence is implemented first, followed by a measurement phase M0 (comprising two measurement sub-phases M01, M02; M01 for a measurement pulse (measurement preparation) and M02 for a single measurement) of a preceding measurement sequence and an inversion phase I2 of a following second measurement sequence within the recovery phase R1 of the first measurement sequence. Following this are a measurement phase M1 (including two measurement sub-phases M11, M12; M11 for a measurement pulse (measurement preparation) and M12 for a single measurement) of the first measurement sequence and an inversion phase I3 of a following third measurement sequence within the recovery phase R2 of the second measurement sequence. Following this are a measurement phase M2 (comprising two measurement sub-phases M21, M22; M21 for a measurement pulse (measurement preparation) and M22 for a single measurement) of the second measurement sequence and an inversion phase I4 of a following fourth measurement sequence within the recovery phase R3 of the third measurement sequence. A measurement phase M3 (including two measurement sub-phases M31, M32; M31 for a measurement pulse (measurement preparation) and M32 for a single measurement) of the fourth measurement sequence and an inversion phase I5 of a following fifth measurement sequence follow within the recovery phase R4 of the fourth measurement sequence, the said fifth measurement sequence not being shown in FIG. 4. This scheme is periodically repeated until all resonance data of all slice planes are acquired. The measurement sequences from FIG. 4 correspond to the exemplary pattern of a measurement sequence as illustrated in FIG. 1.
Only 10 time units are thus required given a distributed acquisition scheme for implementation of two individual measurements. Relative to the sequential acquisition scheme, a distinct shortening of the total measurement duration of an examination can inasmuch be achieved both via the use of the distributed acquisition scheme and via the use of the overlapping acquisition scheme.
Notwithstanding, the distributed acquisition scheme is plagued with the disadvantage that the efficiency of the utilization of the recovery phases depends on the ratio of the sum made up of time duration of the inversion phase of the preceding measurement sequence and time durations of the individual measurements of the subsequent measurement sequence to the time duration of the recovery phase of a considered measurement sequence. If relatively few individual measurements or comparably short individual measurements are implemented, such that the summed time duration of the time durations of the individual measurements is relatively small, the efficiency is also comparably poor, and vice versa. The time duration of an examination inasmuch significantly depends on the selected examination subject and the parameters selected for this purpose.